1. Field of the Invention
The invention relates to the production of an attenuated virus of transmissible gastroenteritis of swine (TGE), the vaccine resulting from said attenuated virus and the administration of said vaccine.
2. The Prior Art
Recently published reports on the mechanism of passive immunity against TGE virus, indicated that there is a marked advantage of oral in contrast to parenteral vaccination of pregnant sows for providing immunity to suckling piglets. The secretion of TGE antibodies of the immunoglobulin (IgA) class in the milk as a result of oral but not parenteral (except intramammary route of vaccination) vaccination was considered to be the most appropriate explanation. Attempts to stimulate the production of such antibodies in the milk have been the most successful as a consequence of an infection of the gastrointestinal tract with natural or attenuated virus. The explanation has been proposed that, after antigenic stimulation of the gastrointestinal tract of pregnant sows with live attenuated organ-cultured TGE virus, sensitized immunocytes from the lamina propria relocate and colonize the mammary gland where antibodies of the secretory IgA class are synthesized and secreted in colostrum and in the milk.
TGE vaccines that were available have not been completely satisfactory for controlling the disease in the field and of very limited value. These vaccines did not produce adequate passive immunity. The TGE virus modified by numerous passages on pig kidney tissue culture can rapidly be reversed in its virulence in one to three back passages. Thus, it was hard to achieve the right type of attenuation which will satisfy both safety and immunogenicity.